Mauntel, TC, Begalle, RL, Cram, TR, Frank, BS, Hirth, CJ, Blackburn, T, and Padua, DA. The effects of lower extremity muscle activation and passive range of motion on single leg squat performance. J Strength Cond Res 27(7): 1813–1823, 2013—Knee valgus is a potential risk factor for lower extremity (LE) injuries. Clinical movement screenings and passive range of motion (PROM) measurements may help identify neuromuscular patterns, which contribute to knee valgus. The purpose of this study was to compare LE muscle activation and PROM between subjects who display visual medial knee displacement (MKD) during a single leg squat (SLS) and those who do not. We hypothesized that muscular activation and PROM would differ between the groups. Forty physically active adults (20 controls, 20 MKDs) participated in this study. Subjects completed 10 LE PROM assessments and performed 5 SLS trials while electromyography (EMG) data were collected from 8 LE muscles. Three separate multivariate analysis of variance were used to identify group differences in EMG data, muscle coactivation, and PROM. Results during the SLS indicated hip coactivation ratios revealed smaller gluteus medius to hip adductor (GMed:Hip Add) (p = 0.028) and gluteus maximus to hip adductor (GMax:Hip Add) coactivation ratios (p = 0.007) compared with the control group. Also, the MKD group displayed significantly less passive ankle dorsiflexion with the knee extended (p = 0.047) and flexed (p = 0.034), and greater talar glide motion (p = 0.012). The findings of this study indicate that MKD during a SLS seems to be influenced by decreased coactivation of the gluteal to the hip adductor muscles and restricted dorsiflexion. Therefore, conditioning, rehabilitation, and injury prevention programs should focus on decreasing hip adductor activity, increasing hip abductor and external rotator activity, and increasing ankle dorsiflexion in hopes to decrease the incidence of these injuries.